The quantitation of light emitted by a sample, such as by luminescence or fluorescence, provides a useful method of analyzing a sample under a wide range of applications. Examples of applications where the emitted light may be used to analyze the sample include, but are not limited to, genetic reporting, enzyme assays, immunoassays, the quantitation of DNA proteins, detection of antigens, and the identification of a sample or the composition of a sample. Luminescence and fluorescence assays, including bioluminescence and chemiluminescence, are extremely sensitive such that any stray light intercepted by the detection system may have a significant impact upon the results. Thus, a system for accurately, consistently and reliably measuring the amount of light emitted from a sample is desirable.
Luminescence assays and the like are often conducted using a sample tray such as a microwell plate which holds a plurality of samples for increased throughput. With the microwell plate and the like, samples are retained in a plurality of closely spaced wells formed in the plate. The samples in the nearby wells provide a source of stray light which, given the sensitivity of the assays, may interfere with the ability to accurately quantitate the light emitted from a selected sample. This crosstalk phenomena is particularly damaging to the assay results when a sample exhibiting low luminescence is located in the vicinity of a sample with high luminescence, with the bright sample essentially preventing any meaningful reading of the adjacent low-light sample.
One method of reading a sample plate utilizes a photodetector which reads one sample well at a time, with the photodetector and/or the sample tray being moved to successively align the photodetector with each of the wells in the plate. U.S. Pat. No. 5,202,091 shows an example of a system in which the tray is moved to align individual wells with an aperture formed in a top plate of the sample chamber. The photodetector reads the sample through the aperture in the top plate. Since only the selected well is visible, the top plate may reduce crosstalk interference from the adjacent wells. However, moving the tray to precisely align each well with the aperture is complicated and time consuming. Moreover, sample luminescent intensity can change over time. Because of the amount of time required to individually read the samples, accurate measurements of the later samples may not be obtained because of sample decay over time.
U.S. Pat. No. 5,290,513 shows a system in which the tray is moved back and forth in one direction and the photodetector is moved back and forth in a direction perpendicular to the motion of the tray. Although the movement of the tray is simplified compared to the system of U.S. Pat. No. 5,202,091, requiring movement of both the tray and the photodetector increases the complexity of the system. U.S. Pat. No. 5,401,465 shows another example of a system in which the sample tray is moved relative to the photodetector. The photodetector is biased by springs against the sample tray to close the gap between the upper edges of the wells and the aperture of the photodetector.
In the system shown in U.S. Pat. No. 5,202,091, the tray is held within a box-like holder. The top of the holder is a mask plate formed with a plurality of apertures. Each of the wells in the sample tray is aligned with one of the apertures in the mask plate to expose the samples retained within the tray holder. The mask plate provides a means of viewing the samples enclosed within the tray holder, but does not eliminate crosstalk effects since every well is exposed through the plate. U.S. Pat. No. 5,611,994 shows another example of a system in which the tray holder is moved relative to a fixed optical system. The tray holder includes a top mask plate which is pivotally mounted to the body of the tray holder, with apertures in the mask plate being aligned with each of the sample wells in the tray. The tray of the system shown in U.S. Pat. No. 5,290,513 is pressed against a diaphragm plate to reduce crosstalk effects between adjacent sample containers. The stationary diaphragm plate is formed with a row of holes which are aligned with one row of sample containers in the tray. The photodetector reads the samples through the holes in the diaphragm plate. Since an entire row of sample containers are exposed through the diaphragm plate, the adjacent samples may interfere with the reading. A system for quantitating light emitted from a plurality of samples which minimizes or eliminates crosstalk between adjacent samples is desirable.
U.S. Pat. No. 4,710,031 shows an example of a microwell plate reader which includes a light source positioned below the microwell plate for illuminating the wells. A transparent overlay with a plurality of black dots is positioned between the light source and the plate. A first reading of the sample is made when the transparent sections of the overlay between the black dots are aligned with the wells so that the light source illuminates the samples. For the second reading, the samples are viewed against a black background provided by moving the overlay to align a black dot with each well. The system may be used to examine the precipitates or suspended material of a sample in front of a black background using side lighting, and then to examine the colors of the samples by illuminated the samples from below.
A system for the efficient measurement of light emitted by a plurality of samples is desirable. A quantitation system which minimizes or eliminates crosstalk effects is also desirable. Similarly, a quantitation system which may be easily and inexpensively manufactured and maintained and efficiently operated to consistently achieve reliable results is desirable.